where \(a, b, c\)  are positive constants, and \(Z_1, Z_2, \ldots \)  are i.i.d. with mean zero. Unlike (REF ), the model (REF ) has only one innovation white noise sequence. Its volatility term does not form a separate time series; instead, it depends on previous values \(X_{t-1}\)  of the observed process \(X\) . The model (REF ) is the classic benchmark for heteroscedastic financial time series. See [1]} or [2]}.
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